Method for producing whiskers

ABSTRACT

1. A METHOD OF GROWIN CARBIDE WHISKERS OF SILICON AND OF BORON COMPRISING THE STEPS OF PREPARING AN INTERDISPERSION OF MATERISLS REACTABLE TO FORM CARBIDE WHISKERS IN TH PRESENCE OF A SUITABLE CATALYST, ALL MATERIALS INCLUDING CATALYST BEING IN THE FORM OF GRANULES RANGING IN SIZE FROM 1/200U TO 5U, SAID INTERDISPERSION INCLUDING CARBON IN THE FORM OF A MEMBER OF THE GROUP CONSISTING OF GRAPHITE, CARBON BLACK, CHARCOAL AND A CARBOHYDRATE THERMALLY DECOMPOSABLE TO CARBON AT SUFFICIENTLY HIGH TEMPERATURE, SAID CATALYST BEING SELECTED FROM THE GROUP CONSISTING OF NI AND COMPOUNDS THEREOF REDUCIBLE TO NI WHEN EXPOSED OF H2 AT HIGH TEMPERATURE WHEN BORON CARBIDE IS TO BE FORMED, AND SAID CATALYST BEING SELECTED FROSM THE GROUP CONSISTING OF NI, PD, CO AND FE COMPOUND REDUCIBLE TO NI, PD, CO, OR FE IN THE PRESENCE OF H2 AT HIGH TEMPERATURE, WO3 AND MIO2, WHEN SILICON CARBIDE IS TO BE FORMED, SAID CATALYST FOR PREPARING WHISKERS OF EITHER BORON OR SILICON CARBIDE BEING PRESENT IN QUANTITY FROM 1X10**-3 TO 2 ATOMIC PERCENT, AND AN ELEMENT SELECTED FROM THE GROUP CONSISTING OF SI AND B OF WHICH A CARBIDE IS TO BE FORMED, SAID ELEMENT BEING PRESENT IN BOTH THE ELEMENTARY AND AN OXIDE FORM, AND EXPOSING SAID INTERDISPERSION TO HYDROGEN AT ELEVATED TEMPERATURE FOR A PEROD OF TIME SUFFICIENT TO ACHIEVE THE DESIRED GROWTH.   D R A W I N G

United States Patent O Im. C1. cdn, 31/36 U.S. Cl. 423-291 8 ClaimsABSTRACT OF THE DISCLOSURE A method of growing single-crystal whiskersof a variety of types from granular interdispersions, eachinterdispersion containing a catalyst and a component which can producethe whisker material when exposed to hydrogen at elevated temperature.It is essential that the components be in granular form and that thecatalyst content lie within stated limits. The method is suitable forlarge-Scale production.

CROSS-REFERENCE TO RELATED APPLICATION This application is acontinuation-in-part application of our co-pending application havingthe Ser. No. 888,771, filed Dec. 29, 1969, now abandoned.

BACKGRO'UND OF THE INVENTION Whiskers of Various types, including metalsand carbon as well as metal carbides, have received considerableattention, largely because of the .fact that they exhibit strengthswhich are close to the theoretical as based on the strength of bondsbetween individual atoms. Although a number of types of whiskers orfilaments are already marketed commercially, the prices of such whiskersare extremely high due to the difiiculty of producing them in quantity.Moreover, with respect to certain types of whiskers, reliable methods ofproducing whiskers substantially uniform in strength, diameter andlength have not been available. A number of techniques have been studiedin the attempt to produce single crystals of various materials inquantity. According to one theory, single crystals are produced as theresult of a heterogeneous reaction involving solid, vapor and liquidphases. However, reactions have been shown to take place by growth froma liquid phase such as in U.S. 3,060,013, in which Harvey disclosesgrowth of copper from a layer of molten lead in which presumably copperis dissolved. Fullman et al., in U.S. 2,842,469, disclose a method ofmaking single crystals of iron in which ammonium halide must be presentin the atmosphere. Mayer et al., in U.S. 3,063,866, form bismuthwhiskers by a process which involves vacuurn deposition.

Using techniques such as those described, it is difficult to ascertainthe effect of impurities which might act as catalysts. Similarly, theeffects of particle Shape, particle size and distribution on the rate ofgrowth and the type of whiskers produced cannot readily be discerned.Furthermore, where a three-phase process is involved, it becomesdifficult to determine whether conditions in one phase rather than inanother are controlling as to the type and rate of whisker growthproduced.

SUMMARY OF THE INVENTION Single-crystal whiskers of a metal or a carbideare grown by subjecting a composition including a hydrogenreducibleeompound of the metal and a suitable catalyst to the effect of hydrogenat elevated temperature. To produce carbide whiskers the compositionmust contain carbon as well as the hydrogen-reducible eompound of themetal and a catalyst. It is essential that the compositions consist ofwell-interdispersed granules. Suitable catalysts are Ni, Fe, Co, W andPd as metals or as solid compounds in granular form.

Accordingly, an object of the present invention is to provide animproved method of producing single-crystal whiskers of a metal or acarbide.

Another object of the present invention is to provide an improved methodof producing single-crystal whiskers of a metal or a carbide in largequantities.

A further object of the present invention is to provide an improvedmethod of producing single-crystal whiskers of Controlled diameter andlength.

Still another object of the present invention is to provide an improvedmethod of producing single-crystal whiskers of a metal or a carbideusing compositions in granular form.

Yet another object of the present invention is to provide an improvedmethod of producing single-crystal whiskers of a metal or a carbidewherein a catalyst is used, the concentration of catalyst being withineffective limits.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

The invention accordingly comprises the several steps and the relationof one or more of such steps with respect to each of the others thereofwhich will be exemplified in the method hereinafter disclosed, and thescope of the invention will be indicated in the claims.

BRIEF DESCRIPTION O'F THE DRAWING For a fuller understanding of theinvention, reference is had to the following description taken inconnection with the accompauying drawing, in which:

The single figure shows the activation energy for whisker formation inthe presence of and in the absence of a catalyst.

DESCRIPTION OFv THE PREFERRED' EMBODIMENTS In accordance with thepresent invention, it has been found that catalysts in granular formthoroughly interdispersed with reactive materials from which whiskersare to be produced can act as seeds which are effective for the growthof whiskers at high rate and in high quantity. For the catalyst to beeffective, the granular size must be between about l/goop and 5a. If thediameter of a catalyst particle is less than about /goop., then theparticle is ineffective for producing whisker growth; conversely, if theparticle is greater in diameter than about 5a, then it can actuallyimpede the growth of whiskers. In fact, little advantage is gained byhaving the catalyst particle size greater than about 2a.

The method by which a catalyst acts in the growth of whiskers isillustrated in the single figure where curve 1 shows the potentialbarrier which must be traversed in transforming a granular material intoa whisker. Curve 2 shows the potential barrier when a catalyst is used.The activation energy for the two reactions is AE1 and AE2,respectively. Where AEZ is much smaller than AE1, as in the presentcase, a far larger percentage of the atoms or molecules involved havethe requisite energy to traverse the potential barrier and consequentlythe reaction is much more rapid.

In the method of the present invention, both the reactants and thecatalysts are in the form of fine granules; consequently, by thoroughlyinterdspersing the catalyst particles and the reactant particles, thearea of contact between the different solid phases is great and thedesired reaction for the production of whiskers is effectivelycatalyzed. Following are examples showing how catalysts are used in theproduction of whiskers in accordance with the present invention: a

EXAMPLE 1 Raw materials: Grams used [W03 500 NiClz (as catalyst) 0.5

W03 and NiClz either separately or together are reduced in size todiameters ranging from about 1,210 to a and thoroughly interdispersed asby rolling in a mill. The interdispersed solid granules are placed in ahydrogen atmosphere where hydrogen gas is flowing at a rate of 100 to1,000 liters/hour and the temperature is raised to between 950 and 1100C. The system is kept under these conditions until the growth ofwhiskers is observed to be complete, or if desired, the growth can beterminated by decreasing the temperature.

Using a flow rate of 300 liters of hydrogen/hour at a temperature of1,000 C., a crop of about 350 grams of whiskers having a diameter of 0.5to lp. and a length of 20 to 200p is obtained.

EXAMPLE 2 Raw materials: Grams used W03 500 PdCl2 (as the catalyst) 0.5

EXAMPLE 3 Raw materials: Grams used Wo3 500 Pt powder (as the catalyst)0.5

The procedure is eXactly as in Example 1. With a flow rate of 300 litersof hydrogen per hour at a temperature of 1,000 C., a crop of 200 gramsof tungsten whiskers having a diameter of 1 to 3,11. and a length of 20to 100/r is obtained.

EXAMPLE 4 Raw materials: Grams used M002 500 NiCl2 (as the catalyst) 0.5

An interdispersion of M002 and NiClz prepared as above, is placed in ahydrogen atmosphere with the flow rate of hydrogen gas through thecontainer 'being at 100 to 1,000 liters/hour and the temperature israised to between 800 and 1l00 C. The temperature and quantity ofhydrogen used will alfect the diameter and length of the molybdenumwhiskers produced. With a -fiow rate of 300 liters of hydrogen per hourat a temperature of 900 C. a crop of about 250 grams of molybdenumwhiskers having a diameter of 0.5 to 2,11. and a length of to 100p. isobtained. f

Palladium and platinum may be used either as the metal or as thechloride in the same manner as the nickel chloride in the presentexample.

EXAMPLE 5 Raw materials: Grams used Si 40 S102 10 Graphite 25 FeCl3 (asthe catalyst) 0.5

A mixture of granular Si, SiOz, graphite, and FeCl3 in the size rangespecified in Example 1 is placed in an atmosphere of hydrogen in anelectric furnace and the temperature is raised to between 1300 and 1600C. When the rate of flow of hydrogen gas is 50 liters/hour at atemperature of 1400 C. a large quantity of silicon carbide whiskers areobtained having a diameter of 0.5 to 2,11. and a length of 5 to 50a.

4 EXAMPLE 6 Raw materials: Grams used Si 40 SiOz 10 Graphite 25 NiClz(as the catalyst) 0.5

A mixture of granular Si, SiOz, graphite and NiClz in the particle sizerange specified in Example 1 is placed in an electric furnace and heatedin a hydrogen atmosphere at a temperature between 1300 and l600 C. Whenthe reaction is carried out in the presence of the flow of hydrogen gasof 50 liters/hour at a temperature of 1400 C. a large quantity ofsilicon carbide whiskers having a diameter of 0.1 to 1 micron and alength of 5 to 30 microns is obtained. Palladium may be used as thecatalyst in place of nickel with essentially the same results so far asshe quantity and size of carbide whiskers is concerned.

EXAMPLE 7 Raw materials: Grams used Si 40 SiOz 10 Graphite 25 CoCl3 (asthe catalyst) 0.5

The granular interdispersion of Si, SiOz, graphite and CoCl3 havingparticle sizes in the range specified in Example 1 is placed in anelectric furnace and heated in a hydrogen atmosphere at a temperaturebetween 1300 and 1600 C. Cobalt is less effective as the Catalyst forthe growth of silicon carbide whiskers than are iron, nickel andpalladium catalysts. When the reaction is carried out in a flow ofhydrogen at 50 liters per hour at a temperature of 1400 C. a relativelysmall crop of silicon carbide whiskers having a diameter of 1 to 2microns and length of 5 to 20- microns is obtained.

Manganese is about as effective as cobalt as a Catalyst.

EXAMPLE 8 Raw materials: Grams used Si 40 Si02 10 Graphite 25 W03 0.5

A granular interdispersion of Si, Si02, graphite and W03 in the sizerange specified in Example 1 is put into an electrical furnace andheated in a hydrogen atmosphere at a temperature in the range of 1300 to1600 C. When the reaction is carried out in an atmosphere of hydrogenflowing at a rate of 50 liters per hour at a temperature of 1400L7 C., alarge quantity of silicon carbide whiskers having a diameter of 0.1 to 1micron and a length of 5 to 50 microns is obtained. Molybdenum oxide isabout as effective as tungsten oxide as the catalyst in this particularreaction.

EXAMPLE 9 Raw materials: Grams used B 40 E203 Graphite 20 NiCl2 (as thecatalyst) 0.5

A granular interdispersion of B, E203, graphite and NiClz having a sizerange within that specified in Example 1 is placed in an electricfurnace and heated in an atmosphere of hydrogen at a temperature between1300 and 1600 C. When the reaction is carried out at a fiow of 50 litersof hydrogen per hour at a temperature of 1450 C. boron carbide (B4C)whiskers having a diameter of 0.1 to l micron and a length of 5 to 30microns are obtained. Iron, palladium, tungsten and molybdenum are aboutas effective as nickel for this particular reaction.

In Examples 5 and 6, SiO can be substituted for SiO2.

In Examples 5, 6, 7, 8 and 9, the reaction can be carried out in asealed chamber initially containing air or hydrogen. The oxygen in theair will, of course, be converted into carbon monoxide and carbondioxide as the temperature is raised and finally as it is lowered. Atthe peak temperature, of course, carbon dioxide is unstable.

In Examples 5 through 9, charcoal, carbon black or a carbohydrate maysubstituted for graphite. Where a car- 5 bohydrate is used, it isdesirable to increase the quantity so that the carbon content in thesystem is the same as if one of the pure carbon materials were used.

In the method disclosed in the present invention, the catalyst controlsboth the reaction and the formation of the whiskers. The catalyst may beadded as elements or as compounds which can be reduced either byreaction by the raw materials or by the hydrogen atmosphere specified.For example, nickel halide catalysts for the formation of tungstenwhiskers are all reduced to metallic nickel by hydrogen gas. However, itshould be noted that where the catalysts are added as compounds, contactbetween the granules of the different materials may be less extensivewhich can have an effect on the rate of growth of whiskers.

In general, the quantity of catalyst used is best expressed in terms ofatomic percentage based on the material of which the whiskers arecomposed. For example, the quantity of catalyst for the preparation oftungsten or molybdenum whiskers should be between 1 l 3 atomic percentand 2 atomic percent. If the quantity of catalyst is less than about 110-3 atomic percent whiskers are rarely produced. On the other hand, ifthe quantity of catalyst is greater than about 2 atomic percent, theqauntity of whiskers produced is less and the tungsten and molybdenumwhiskers tend to be angular. The lower limit for the quantity ofcatalyst to be used in the production of SiC or B4C is again about1x10-3 atomic percent. The upper limit is about 5 atomic percent. Thequantity of whiskers produced decreases rapidly if this content isexceeded.

As is evident from the above remarks, the most effective range ingeneral is 1 10-3 to 5 atomic percent for the quantity of catalyst basedon the principal element in the whiskers produced. Apparently, thecatalyst condenses either at the top or the bottom of whiskers as thewhiskers grow. As a result, the use of catalysts in the form given isalso suitable where vapor mechanisms and liquid mechanisms are involved.

The Examples presented include tungsten whiskers, molybdenum whiskers,silicon carbide whiskers and boron carbide whiskers. However, as isapparent, the methods of the present invention can be applied to theformation of whiskers of other materials as well, provided that thereactants can be prepared in the proper size range and thoroughlyinterdispersed. Testing has shown that whiskers prepared in accordancewith the present invention are as effective for the reinforcement ofvarious composites as are those produced by prior methods. Furthermore,growth is sufliciently rapid and the yields are sufficiently great sothat the cost of whiskers produced by the present methods issubstantially lower.

It should be noted that all of the catalysts specified are eithertransition metals or compounds thereof. It is reasonable to expect thatall transition metals or compounds thereof will show similar catalyticeffect.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efi'iciently attained and,since certain changes may be made in carrying out the above methodwithout departing from the spirit and scope of the invention, it isintended that all matter contained in the above description shall beinterpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

What is claimed is:

1. A method of growing carbide whiskers of silicon and of boroncomprising the steps of preparing an interdispersion of materialsreactable to form carbide whiskers in the presence of a suitablecatalyst, all materials including catalyst being in the form of granulesranging in size from Vzoou to 5,41., said interdispersion includingcarbon in the form of a member of the group consisting of graphite,carbon black, charcoal and a carbohydrate thermally decomposable tocarbon at sufliciently high temperature, said catalyst being selectedfrom the group consisting of Ni and compounds thereof reducible to Niwhen exposed to Hz at high temperature when boron carbide is to beformed, and said catalyst being selected from the group consisting ofNi, Pd, Co and Fe compounds reducible to Ni, Pd, Co, or Fe in thepresence of Hz at high temperature, W03 and MOz, when Silicon carbide isto be formed, said catalyst for preparing whiskers of either boron orsilicon carbide being present in quantity from 1 x10-3 to 2 atomicpercent, and an element selected from the group consisting of Si and Bof which a carbide is to be formed, said element being present in boththe elementary and an oxide form, and exposing said interdispersion tohydrogen at elevated temperature for a period of time sufiicient toachieve the desired growth.

2. The method as described in claim 1 wherein said interdispersionconsists essentially of 40 parts by weight of Si, 10 parts by weight ofSiO, 25 parts by weight of graphite and 0.5 parts by weight of acatalyst selected from the group consisting of NiCl2, Pd and C0013.

3. The method as described in claim 1, wherein said whisker produced isboron carbide and said solid mixture consists of B, E203, and a materialselected from the group consisting of graphite, charcoal, carbon blackand a carbohydrate, all in -granular form.

4. The method las described in claim 1, wherein said whisker produced isa member selected from the group consisting of silicon carbide and boroncarbide and said elevated temperature lies between 1300 C. and 1600*' C.

5. The method as described in claim 1, wherein said 'hydrogen in saidhydrogen atmosphere is moved over said interdispersion at a rate ofabout 100 to 1,000 liters per hour for a batch of interdispersedgranular materials weighiug about 500 grams.

6. The method as described in claim 1, wherein said interdispersionconsists essentially of 40 grams of Si, 10 grams of SiO-z, 25 grams ofgraphite and 0.5 'grams of FeCl3.

7. The method as described in claim 1, wherein said interdispersionconsists essentially of 40 grams of Si, 10 Agrams of SiOz, 25 grams ofgraphite and 0.5 grams of C0cl3.

8. The method as described in claim 1, wherein 'said interdispersionconsists essentially of 40 grams of B, 10 grams of E203, 20 grams ofgraphite and 0.5 grams of NiClz.

References Cited UNITED STATES PATENTS 3,409,395 11/1968 Shyne et al.423-345 3,423,179 1/ 1969 Webb 423-291 2,813,811 11/1957 Sears 148-1.62,8l3,811 11/1957 Sears 148-1.6 2,842,468 7/1958 Brenner 148-1.62,842,469 7/1958 Fullman et al 148-1.6 3,147.085 9/1964- Gatti 148-1.6 X3,201,665 8/1965 Venables 148-1.6 X

L. DEWAYNE RUTLEDGE, Primary Examiner I. M. DAVIS, Assistant ExaminerU.s. cl. x.R. i

1. A METHOD OF GROWIN CARBIDE WHISKERS OF SILICON AND OF BORONCOMPRISING THE STEPS OF PREPARING AN INTERDISPERSION OF MATERISLSREACTABLE TO FORM CARBIDE WHISKERS IN TH PRESENCE OF A SUITABLECATALYST, ALL MATERIALS INCLUDING CATALYST BEING IN THE FORM OF GRANULESRANGING IN SIZE FROM 1/200U TO 5U, SAID INTERDISPERSION INCLUDING CARBONIN THE FORM OF A MEMBER OF THE GROUP CONSISTING OF GRAPHITE, CARBONBLACK, CHARCOAL AND A CARBOHYDRATE THERMALLY DECOMPOSABLE TO CARBON ATSUFFICIENTLY HIGH TEMPERATURE, SAID CATALYST BEING SELECTED FROM THEGROUP CONSISTING OF NI AND COMPOUNDS THEREOF REDUCIBLE TO NI WHENEXPOSED OF H2 AT HIGH TEMPERATURE WHEN BORON CARBIDE IS TO BE FORMED,AND SAID CATALYST BEING SELECTED FROSM THE GROUP CONSISTING OF NI, PD,CO AND FE COMPOUND REDUCIBLE TO NI, PD, CO, OR FE IN THE PRESENCE OF H2AT HIGH TEMPERATURE, WO3 AND MIO2, WHEN SILICON CARBIDE IS TO BE FORMED,SAID CATALYST FOR PREPARING WHISKERS OF EITHER BORON OR SILICON CARBIDEBEING PRESENT IN QUANTITY FROM 1X10**-3 TO 2 ATOMIC PERCENT, AND ANELEMENT SELECTED FROM THE GROUP CONSISTING OF SI AND B OF WHICH ACARBIDE IS TO BE FORMED, SAID ELEMENT BEING PRESENT IN BOTH THEELEMENTARY AND AN OXIDE FORM, AND EXPOSING SAID INTERDISPERSION TOHYDROGEN AT ELEVATED TEMPERATURE FOR A PEROD OF TIME SUFFICIENT TOACHIEVE THE DESIRED GROWTH.